Roughly 230,000 new cases of prostate cancer are expected in the U.S. this year. Typically 80-90% of these cases are relatively early stage disease for which various treatment options are available. Primary treatment options involving radiation include external beam radiation therapy, which uses high-energy x-ray beams that intersect the prostate from multiple angles, and brachytherapy, in which a radioactive source is introduced into the prostate itself. Typical brachytherapy techniques use so-called “seeds,” which are small (approximately 0.8×4.5 mm) cylinders that contain a radioactive element in a stainless-steel casing. A number of seeds, usually ranging from 80-120 seeds, are placed into the prostate using small gauge needles. The seeds can remain in place permanently while the emitted radiation decays over time. The common radioisotopes used in the seeds are iodine-125, palladium-103 and cesium-131. The goal of the radiation oncologist is to ensure that the total dose received by the cancer cells is sufficient to kill them. Seeds can be placed during an outpatient procedure in a single day and thus present an attractive treatment option for patients versus the many weeks required for external beam radiation therapy. Good candidates for brachytherapy seed therapy are typically patients having a PSA value ≦10, a Gleason score of ≦7 and low-stage disease (T1c or T2a); however, patients with more advanced stage disease may also be treated with brachytherapy. In some cases, patients (e.g., with more advanced disease) may be candidates for brachytherapy plus external beam therapy. The use of seeds has grown rapidly, and long-term survival data for LDR brachytherapy treatment of the prostate is typically good.
In treating prostate cancer with brachytherapy seeds, it may be desirable to create a uniform radiation pattern within the prostate gland or within a region of the prostate gland. Computer code or treatment planning software can be used to select the number of seeds and their relative placement so that the desired radiation dose is achieved. This is a relative complex procedure because each individual seed is essentially a “point source” of radiation, and thus the radiation contributions from all of the seeds must be summed to achieve the total radiation dose. When the seeds are placed, great care is typically taken to ensure that they are arranged in the pattern specified by the treatment planning software. However, some deviation in seed placement may occur due to the divergence of needles as they are inserted. See Nath et al., Med Phys 27, 1058 (2000). A more problematic occurrence is the tendency of seeds to migrate once they exit the insertion needle. See Meigooni et al., Med Phys 31, 3095 (2004). It is not uncommon for seeds to migrate. In some cases, seeds may be caught in an efferent vessel and become embolized in the lung or excreted with urine. Gross movement of the seeds can create non-uniformities in the radiation pattern and thus potentially compromise the efficacy of therapy.
In an attempt to mitigate the post insertion migration of brachytherapy seeds, various products have been developed. For example, the RapidStrand™ device from Oncura (Arlington Heights, Ill., USA) is a hollow suture material that contains conventional seeds in a “linked sausage” arrangement. The suture material subsequently dissolves away leaving the seeds implanted in the patient. However, the seeds are held by the suture for a time that allows for healing and better retention of the seeds. Various so-called “sleeves for seeds” are also available. Another device that is commercially available from IBA (Louvain-la-Neuve, Belgium) under the trade name Radiocoil™ is a coiled structure device that contains rhodium metal that is proton-activated to produce Pd-103. Accordingly, the radioactivity is emitted along the entire length of the device.
Notably, the ability of the radiation oncologist to achieve the highest accuracy in therapy planning is hampered by the discrete nature of the current “seed” radiation sources due to their limited size and anisotropic radiation patterns. The tendency of seeds to move when placed in or near prostatic tissue is a problem that, while not invalidating this excellent form of therapy, creates a non-ideal situation for planning (e.g., requiring revalidation of the placement by CT scan). For example, migration of seeds to the lungs can result in incidental lung doses that are not favorable.